There is an increasing need for mass analysis of fast processes, which in part, arises from the popularity of fast multi-dimensional separations techniques like GC-TOF, Mobility-TOF, or EM-TOF, (electron monochromator) etc. In those methods, the TOF serves as a mass monitor scanning the elution of the analyte of the prior separation methods.
There are numerous other fields of application involving the investigation of fast kinetic processes. Two examples are the chemical processes during gas discharges, and photon or radiofrequency induced chemical and plasma ion etching. In the case of gas discharges one may monitor the time evolution of products before, during and after the abrupt interruption of a continuous gas discharge or during and after the pulsed initiation of the discharge. An analogous monitoring of the chemical processes in a plasma etching chamber can be performed. The time profile of chemical products released from a surface into a plasma can be determined either during and after the irradiation with laser pulses or before, during and after the application of a voltage which induces etching (e.g., RF plasma processing). A third such example is the time evolution of ions either directly desorbed from a surface by energetic beams of X-ray, laser photons, electrons, or ions. In addition, when the ions are desorbed from a surface there is usually a more predominant codesorption of non-ionized neutral elements and molecules whose time evolution can be monitored by first post ionizing neutral species which have been desorbed and then measuring mass separated time evolution of the ions by mass spectrometry. Yet a fourth area of use is the monitoring of the time evolution of neutral elements or molecules reflected after a molecular beam is impinged on a surface. The importance of such studies range from fundamental studies of molecular dynamics at surfaces to the practical application of molecular beam epitaxy to grow single crystalline semiconductor devices. A further application for fast analysis is presented by Fockenberg et al. Yet another application is when the ionized output of multiple separation techniques must be monitored simultaneously. For example, one such application could be where the output of several chromatographic columns (e.g., liquid chromatograph, gas chromatograph) are each coupled to an ionization source (e.g., electrospray, photoionization, electron impact). The readout of each column must then be fluidly coupled to an individual mass spectrometer.
In all such studies the time evolution of ion signals which have been mass resolved in a mass spectrometer is crucial. TOF instruments have become the instrument of choice for broad range mass analysis of fast processes.
TOF instruments typically operate in a semi-continuous repetitive mode. In each cycle of a typical instrument, ions are first generated and extracted from an ion source (which can be either continuous or pulsed) and then focused into a parallel beam of ions. This parallel beam is then injected into an extractor section comprising a parallel plate and grid. The ions are allowed to drift into this extractor section for some length of time, typically 5 μs. The ions in the extractor section are then extracted by a high voltage pulse into a drift section followed by reflection by an ion mirror, after which the ions spend additional time in the drift region on their flight to a detector. The time-of-flight of the ions from extraction to detection is recorded and used to identify their mass. Typical times-of-flight of the largest ions of interest are in the range of 20 μs to 200 μs. Hence, the extraction frequencies are usually in the range of 5 kHz to 50 kHz. If an extraction frequency of 50 kHz is used, the TOF is acquiring a full mass spectrum every 20 μs. After each extraction, it takes some finite time for the ions of the primary beam to fill up the extraction chamber. This so-called fill up time is typically relatively shorter for lighter ions as compared to heavier ions because they travel faster in the primary beam. For light ions, the fill up time may be as short as 1 μs whereas for very large ions, the fill up time may exceed the 20 μs between each extraction, and hence those large ions never completely fill up the extraction region. The fill up time depends on the ion energy in the primary beam, the length of the extraction region and the mass of the ions.
Some fast processes, however, require monitoring with a time resolution in the microsecond range. For example, a species eluting from an ion mobility spectrometer may elute through the orifice within a time interval of 15 μs. If this species also has a small fill up time it is possible that this elution occurs between two TOF extractions in such a way that the TOF completely misses the eluting species.
Known techniques to solve this problem are based on increasing the extraction frequency. In general, the ion flight time in the TOF section will determine the maximum extraction frequency, shorter flight times yielding higher extraction rates. The ion flight time is shortened by either increasing the ion energy in the drift section, or by reducing the length of the drift section. Increasing the ion energy is the preferred method, because decreasing the drift length results in a loss of resolving power. However, because the relationship between ion energy E and the time-of-flight T is a square-root dependence, an increase in energy only leads to a minimal decrease in flight time:
  T  =      a          E      
Thus, more effective methods and corresponding apparatuses for monitoring such fast ion processes while minimizing the loss in sensitivity that occurs when eluted ions are not counted by the detector are needed. In addition, it would be highly desirable if a method of coupling multiple beamlets into one mass spectrometer could be achieved which would allow fast processes in each beamlet to be simultaneously monitored with this one mass spectrometer in a way which would retain a correlation between the time evolution of the mass resolved ions and the individual beamlet from which the ions came. Thus the need for an expensive mass spectrometer to be coupled at the output of each ion beamlet could be eliminated thus significantly reducing the costs for monitoring the time evolution of multiple fast processes.